Centralized traffic controlling system for railroads



2 Sheets-Sheei l 19380 R. M. PHINNEY CENTRALIZED TRAFFIC CONTROLLING SYSTEM FOR RAILROADS Filed March 30, 1957 m L O IPEOU V w/Wnfl VJ 5 2 m .m H Em m% r m: 3 m W S 5 mfi 5 mmz W H E m E w 53mm T 8a mcpr dumvcH 3 mm Fwy/ A fl 20 53 55 .mutQO R. M. PHINNEY CENTRALIZED TRAFFIC CONTROLLING SYSTEM FOR RAILROADS Filed March 30, 1937 2 Sheets-Sheet 2 5E5 PM M my F F 3% nvw m N n Rm m mPfl WM 2 28 Y 6 E V ELBQE M r A J ML K fi t u uz un Q, m u m ma n u mco m fi Patented Oct. 4, 1938 PATENT OFFEQE CENTRALIZED TRAFFEC CONTRQLLENG SYSTEM FOR RAILROADS Robert M. Phinney, Rochester, N. Y., assignor to General Railway Signal Company, Rochester,

Application March 30,

8 Claims.

This invention relates to remote control systems of the type involving a control office and a plurality of remote field stations connected by a single line circuit and it more particularly per- 5, tains to the communication part of a centralized traffic controlling system for railroads.

This invention is in the nature of an improvement over the lookout feature disclosed in Judge Patent 2,045,713 issued June 30, 1936, and I propose to claim only the improvement herein. This invention is also in the nature of an improvement over the application of Preston, Ser. No. 179,982, having an eifective filing date of May 16, 1936, which does not employ a lockout feature but does employ a rectifier in the line circuit, and no claim is made herein to any invention disclosed in said Preston application.

The present invention contemplates a centralized traffic controlling system, in which the switches and signals at a plurality of stations, locate-d along a railroad trackway, are controlled from the control ofiice and their indications are transmitted to the control ofiice, by means of a I two wire line circuit.

One object of the present invention is the provision of a novel lockout arrangement in a centralized traffic controlling system, whereby only one station is given access to the communication system, in the event that two or more stations attempt to send indications at the same time.

Another object of the present invention relates to an improved means for giving preference to the sending of controls from the office, when a station has indications ready for transmission at the same time the omce has controls ready to send.

Other objects and advantages. of the present invention will be hereinafter set forth in the specification and claims and further details will be better understood by referring to the accompanying drawings, which illustrate one method of carrying out the invention by Way of example.

The drawings illustrate in a diagrammatic manner the apparatus and circuits used in connection with the two wire line circuit for the provision of the novel lockout features above mentioned. For convenience in describing the invention in detail, those parts having similar features and functions are designated in the different figures by like reference letters, generally made distinctive either by the use of distinctive exponents representative of their location, or by the use of suitable preceding numerals repre- 1937, Serial No. 133,856

sentative of the order of their operation, and in which:

Fig. 1 illustrates the apparatus and circuits most closely related to the line circuit and located at the control office.

Fig. 2 illustrates the apparatus and circuits most closely related to the line circuit and located at a field station. A small portion of the apparatus and circuits at the end station of the system is illustrated in the right hand portion of Fig. 2, in order to indicate how the two wire line circuit is terminated at the last station of the series. v

The present invention is shown in abbreviated form in the drawings, since it discloses a means for providing the lockout features in a normally deenergized two wire line system of the simplex type, such for example as disclosed in Judge Patent No. 2,045,713. This patent shows a source of current at the ofiice and a separate source of current at each station, reversibly connected to the line for providing outbound coded polar control impulses and inbound coded polar indication impulses.

In describing the present invention, it will be understood that the two wire line circuit of the above mentioned Judge patent is modified as shown in the accompanying drawings to provide the desired operation. For example, the polar CS relays of the Judge patent are replaced by neutral type relays PC and NC, operated in the manner and for the purpose of applying polar control impulses to the line circuit, as shown for example, in Patent No, 2,082,544 dated June 1, 1937.

With the above in mind, it will now be explained how the lookout features of the present invention are provided.

Control ofi'ice equipment-Jim ciintrol office (Fig. 1) includes a quick acting pola\r line relay F, which maintains its contacts in their neutral positions when the line circuit is deenergized, positions its contacts to the right when\a positive impulse is applied to the line from the control ofiice and positions its contacts to the 1e t when a negative impulse is applied to the line f om the control ofiice.

A quick acting line repeating relay Fit of the neutral type repeats each energization of relay F, irrespective of the polarity of the impu J/se which energizes relay F. Slow acting relay SA is en ergized each time relay F operates its contacts to the right or left and, due to its slow acting characteristics, relay SA remains in its picked up position during the intervals between the impulses which are applied to the line circuit. Relay SA is energized at the beginning of a cycle, remains picked up throughout the cycle and is dropped away at the end of the cycle, when the line circuit is deenergized for a comparatively long inlarity of the impulses applied to the line circuit,

by means of their pole changing contacts. Oflice start relay C is picked up at the beginning of a cycle initiated at the control oiiice and is stuck up throughout such a cycle. Field start relay F0 is picked up at the beginning of a cycle initiated from a field station and is stuck up throughout such a cycle. Relay EP provides the time spaced impulses in the line circuit.

The purpose of relay F3 is to transfer the line circuit from the source of current in the ofiice to a closed circuit condition, by being picked up at the start of an indication cycle, so that the line circuit may be energized from the field station source of current. A bank of stepping relays iV, 2V and 3V serve to mark oil the steps of each cycle of operations, that is, a stepping relay is picked up during each time space between impulses for selecting the PC or NC relay for providing the next control impulse. These stepping relays also select'the indication circuits, which are energized in accordance with the position assumed by the polar contacts of relay F during an indication cycle.

' Field station equipment-The field station equipment is similar to the control ofiice equip ment, with certain exceptions now to .be pointed out. For example, relays F F1, SA SE PC 49 N0 F13 and the stepping relays perform functions similar to corresponding relays in the control office, asabove explained. Likewise, the stepping relays illustrated in Fig. 2 select the control circuits to be energized in accordance with the polarity of the control impulsesrepeated bythe position of the lower polar contact of relay F These stepping relays also select the indication circuits which are used to selectively operate relays PC and NC for providing polar indications, in a manner similar to the selection of the PC and NC relays in the oiiice for providing polar controls. It is not believed necessary to illustrate thecircuits for controlling these code selecting relays in detail, since they are fully disclosed and explained in the above mentioned patent and application.

Relays L and LO are provided at the field station for eifecting the lookout operations as will be described in detail. Rectifier or valve RC is for the purpose of permitting current to flow over the line circuit, when the system is initiated,

from the field station, from the source of energy which is normally connected to the line in the control oflice. Resistance unit RS is provided to equalize the line circuit resistance, irrespective of the location of the station which is transmitting. For example, resistance RS at the illustrated station will be of the proper value to corn pensate for the resistance of the line extending from this station to the end of the line, so that the line circuit resistance will be approximately the same during transmission from any field station.

Rectifier valve RC is provided at the end station for the purpose of preventing the line being energized from the source of current in the control ofiice, which is connected to the line in a reverse direction and for permitting the line to be ener'gized when the oflice initiates a cycle by connecting the source of current to the line in a normal direction.

These valves may be of any type desired, such 7 for example, as the copper oxide, the electrolytic,

the vacuum tube, or the like.

It is believed that the nature of the invention, its advantages and characteristic features may be best understood with further description being set forth in a manner relating to certain typical operations of the system, reference being had to the above mentioned Judge patent and the above mentioned British patent for a complete detailed operation of the system, including those features which do not relate to the lockout function.

Operation While the system is at rest, the line circuit is normally deenergized because ofiice line battery LB is connected to the line in such a manner that rectifier RC prevents current flow. The relays and circuits of the system are normally deenergized, with a few exceptions. For example,'relay NC of Fig. 1 is normally energized over a circuit extending from (-1-), back contact 29 of relay C, back contact I! of relay FE and winding of relay NC, to

Manual start-With the system at rest, the control ofiice operator can initiate a cycle for the transmission of controls by actuating the desired control levers (not shown) to their proper positions and then operating the start or station selecting button (not shown). The actuation of the starting button is effective to apply (-1-) energy to the start conductor illustrated in the upper, center portion of Fig. 1. This results in the picking up of relay C over a circuit extending from'(+) applied to the start conductor, back contact; I 2 of relay SB, winding of relay C and back contact l3 of relay FC, to

The picking up of relay C deenergizes relay NC at open back contact 29 and closes a circuit for picking up relay PC which extends from front contact ll! of relay C, back contacts l4, I5 and I6 of relays 3V, 2V and IV respectivelyand winding of relay PC, to The dropping of relay NC and the picking up of relay PC changes the reverse connection from battery LB to a normal connection, so that the line is nowenergized with a impulse over a circuit extending from. the terminal of battery LB, front contact I! of relay PC, back contact [8 of relay NC, back contact N9 of relay FB, winding of relay F, back contact 28 of relay EP, lineconductor,

line relay F back contact I Q9 of relay FB line conductor, winding of relay F back contact 219 of relay F3 valve RC return conductor, back contact 20 of relay NC and front contact 3! of relay PC, to the terminal of battery LB;

' The energization of the line positions the polar contacts of relays F, F F? and the like, to their right hand positions. Relay FP, in the ofiice, is picked up over an obvious circuit closed at contact 2| of relay F and relay FP in the illustrated field station, is picked up over a circuit extending from (-5-), contact l2] of relay F in its right hand dotted position,back contact I40 of relay SB v and winding of relay FP to Contact 2| of relay F also closes an obvious circuit for picking up relay SA and, as above mentioned, relay SA remains picked ,up'during the impulses applied to the line throughout the cycle of operations. Relay SA closes an obvious pick up circuit for relay SB at front contact 23.

At the station, relay L is picked up over a circuit extending from contact 53!) of relay F in its right hand dotted position, back contact l3] of relay SA and winding of relay L to After the picking up of relay SA the pick up circuit of relay L is opened at back contact itl, and a stick circuit is completed for relay L extending from front contact 35 ofrelay SA front contact L36 and winding of relay L to The picking up of relay SA closes an obvious-circuit at its front contact I23 for picking up relay SE Relay SE opens the above described pick up circuit for relay FP at back contact Mi but a substitute circuit for relay FP is completed at front contact H39 of relay L The picking up of relay L and the consequent opening of its back contact I24 opens the pick up circuit for relay FB so that this latter relay will remain down to maintain the line circuit connection intact to the end of the line. The picking up of relay L and the consequent opening of its back contact l2'l, opens the circuit of relay L0 so that this relay cannot be picked up during a control cycle. Similarly, at the end station the picking up of the relay L opens'the pick-up circuit (not shown) for relay L0 and by its front contact shunts rectifier BC to allow a following impulse. of negative polarity to pass through the line circuit.

The line is impulsed and thestepping relays at the oflice and at the stations are operated in synchronism, in the same manner disclosed in the above mentioned. Judge patent and the polarity of energization of the line for each impulse is determined during the preceding time space be tween impulses, the code. sending relays PC and NC being controlled in the same manner disclosed in the above mentioned Patent No. 2,082,544. These polar impulses position contact 539 of relay F (and other similar contacts at other stations) to the required positions for energizing the control circuits, as selected through front contacts |3| and 14! of relay SA front contacts i5! and it! of relay L and the channel selecting circuits of the stepping relays to which the legend controls is applied in Fig. 2. It is believed unnecessary to illustrate this feature of the system or to explain its operation further.

During the operation of relay F in the control ofiice, contact 39 is ineffective, since relay FC is not picked up during a control cycle. When relay SA in the control office picksup and before relay SB is picked up, a stick circuit is closed for maintaining relay C energized during the cycle which extends from front contact Q8 of relay SA, front contact M of relay C, windingof relay C and back contact 53 of relay FC, to

Automatic start-The system may be initiated into a cycle of operations for the transmission of indications by a change in conditions at a field station. For example, the track switch at the station may be changed from one position to another, or a signal may be changed from one condition to another for initiating a field start. The manner of starting the system in response to such a field station change has not been illustrated in the present embodiment, since it may be provided in any desired manner, such for example as illustrated in the above mentioned patents. Such a start condition is efiective to apply to the start conductor illustrated in the lower center portion of Fig. 2 and relay L0 is picked up over a circuit extending from applied to this conductor, back contact H2 of relay SB winding of relay LO 'and back contact I21 of relay L to The picking up of relay LO completes a circuit for energizing the line with a impulse, which circuit extends from. the terminal 'of battery LB, backcontact ll of relay PC, front contact 28 of relay NC, return conductor, front contact HQ of relay L0 valve RC resistor. RS back contact l is of relay'FB winding of relayIE line conductor, back contact 28 of relayEP, winding of relay F. back contact IQ of relayFB, front contact H! of relay NC and back contact3l of relay PC, to the terminal of battery LB. This positions the. polar contacts of the Frelays to the left, following which the line repeater relays and the slow acting relays are picked'up, as previously described, and the system steps thrcugh a cycle of operations as before. In this instance, however, relay L at the station is not picked up, because contact I30 of relay F is not positioned to the right before relay SA is picked up. A stick circuit for relay L0 is completed immediately upon the positioning of contact I30 to the left, which stick circuitextends from'1(+), contact I30 of relay F' in its left hand dotted position, back-contact Ml of relaySA front contact M2 of relay L0 winding of relay L0 and back contact 121 of relay L to 'Whenrelay SA picks up, a permanent stick circuit-is completed for maintaining relay L0 energized throughout the cycle, which stick circuit extends through front contact Hit of relay SA During an indication cycle, contact I38 of relay F is ineffective to energize the control circuits because of open front contacts l5! and NH of relay L During an indication cycle, contact 35 of relay F in the control office is eflective t'o energize the local indication circuits in accordance with the polarity of the indication impulses, because of closed front contacts 42 and 43 of relay SA and closed front contacts 44 and 450i relay FC. I

The en rgization of the line closes aicir-' cuit for picking up relay FC in thecontrol office which extends from contact 36 of relay Fin its left hand dotted position, back contact 43. of relay SA and Winding of relay FC, to When relay SA ispicked up, the pick up circuit for relay FC is opened at back contact 43 but a stickcircuit is closed which extends from. front contact 35 of relay SA, front contact 36 and winding of relay FC, to

Relay FB at the station is picked up during the first time space, following the first energization of the line, over a circuit extending from front contact l2?! of relay SE frcnt'contact I22 of relay L0 back contact lid of relay L back contact 5225 of relay and winding of relay FBLf-o During the following cycle, relay FB is. stuck up 'over' a circuit extending from front contact l2ll of relay SE front contact 122 of'relay L0 back contact l24of relay L front contact F28 and winding of relay FB to Relay F13 remaining picked up throughout the cycle, completes the circuit by way of its front-contact M9 to the pole changing contacts of relays P0 and NC so that these contacts are effective to apply polar impulses to the line from the source of current L33 Referring to the control office, relay FB is like wise picked up during the first time space, following the first impulse, over a circuit extending from front contact '25 of relay SB, back contact 2 of relay 0, front contact'ZE 'of relay F0, back contact 22 of relay F? and winding of relay F3, to Relay FB is stuck up throughout an indication cycle over an obvious circuit completed at its front contact 21, Which'circuit is .similar to that traced for the stick circuit. for relay F3 The closure of front contacts !9 of relay FB completes a loop circuit for the line in the control office and the opening of back contact I9 disconnects the circuit of the line which leads to the office code sending relays.

The following cycle is therefore effective for receiving'polar indications in the oflice and energizing the local indication circuits in accordance with the positions of contact 39 of relay F. It will be obvious that the picking up of relay 5E and the consequent opening of its back contact I I drops relay NC, so that this relay remains deenergized throughout an indication cycle. The opening of back contact I3 of relay FC, removes energy from ofiice start relay 0, so that an office start condition will be ineffective, in the event that the operator attempts to initiate a control cycle from the office during a cycle which is transmitting indications.

A plurality of stored start conditions.-As already explained, the FB relays can only be picked up during the time space following the first impulse. This means thatthe line circuit connections, including contacts 19 and HS, will remain as illustrated in the drawings during the first or initiating impulse, and during this initiating impulse conditions will be established for determining the direction of transmission. 7

For example; it will be assumed that the office and the illustrated field station attempt to initiate the'system at substantially the same time. If the office start is sufficiently in advance of the field start to pick up relay C, drop relay NC, pick up relay PC, position relay F to the right and pickup relay L then the field start'condition is ineffective because of open back contact I21 of relay L Furthermore, since the dropping of relay NC removes negative energy from the line, even though relay L0 is picked up it cannot be stuck up because contact I30 of relay F will not go to the left, consequently relay L0 will drop away when relay L picks up and opens it back contact 121..

If the field start is sufficiently in advance'of the ofiice start to pick up relay LO ,'position relay F to the left and pick up relay FC, then the office start condition is ineffective because of open back A contact I3 of relay FC.

If relays C and L0 are picked up at the sam time or in rapid succession, then the office takes precedence because the circuit of relay C is opened in the initiation period only by the picking up of relay F0, and FC cannot pick up unless the line is first energized with a negative impulse. The picking up of relay C drops relay NC, which prevents the negative energization of the line during the initiating period.

,On the other hand, if the station can energize the line with a negative impulse long enough to pick up relay PC, then the station takes precedence, because relay C cannot be energized. If the start conditions are such that relays C and FC are both picked up at substantially the same time, then relay C will drop because of open back contact l3 and keep relay NC energized to position contact 30 to the left for holding FC up, which in turn holds relay C open, thus giving the station preference.

Lockout between field sta.tions.In the event that changes have takenplace at two or more stations simultaneously or in rapid succession, so that more than one field station is ready to transof Fig. 2 is picked up and that a corresponding.

lockout relay at the next station farther-out the line is picked up, the line will be energized withnegative energy by way of valve RC as previously described. The path through this valve is a shunt across the line, which may or may ,not be sufiicient to prevent the operation of the line relay at the more distant station. If this shunt is sufiicient to prevent the distant line relay from operation, then this distant stationis immediately locked out because its associated line repeating and slow acting relays will not be operated. Then when relay FB of the preferred station is picked up, during the time space following the initiating impulse,- the line extending from-the preferred station to the more distant station is opened at back contact H9, thus looking out .the distant station during the following cycle.

If the shunt is ineffective to prevent the operation of the distant line relay, then its associated line repeating and slow acting relays will. be picked up during the initiating period, but when relay F12 at the preferred station is picked up, during the time space following the initiating period, the opening of back contactllS opens up the line outward from the preferred station, thus dropping out the line, line repeater and slow acting relays at the inferiorstation. The preferred station will then transmit its indications during the following cycle in the manner disclosed in the above mentioned patents. j At the inferior station, the lookout relay is stuck up while the associated SA relay is up, by means of a circuit similar'to that including front contact I43 of relay SA of Fig. 2. During the interval between the dropping of the SA and SB relays, however, this stick circuit is opened and the lockout relay is dropped out. At the superior station, contact M3 maintains the stick circuit of relay LO complete until the end of the cycle.

The invention of this application has certain advantages over the system disclosed in the Judge Patent No. 2,045,? 13, in that the look-out arrangement is such that the same source of current,

namely the ofiice battery, is used for rendering either the office or a field station effective. For

instance, if afield station is ready to transmit'an indication it allows current of normal polarity to flow from the office battery, as by shunting the rectifier RC whereas if the ofiice is ready to transmit a control it reverses the polarity of the same ofiice battery and causes current of the opposite polarity to flow in the linecircuit to thereby lock out all field stations and allow the office to transmit. I Obviously, since the same source is used to cause current to flow of one polarity or another no conflict can occur. In the system of the Judge patent a field battery and the office battery may be simultaneously applied to the line circuit resulting in a conflict of two batteries opposing each other, and a suitable struction no such conflict of two batteries oppos- 7.5.

ing each other can occur. This application also distinguishes from numerous applications and patents of common ownership by the provision of a potential normally applied to a line circuit closed for current flow in the opposite direction and without allowing the flow of current from said potential, thus having the advantage of the presence of a potential without the expense of a continuous current flow.

It is not believed necessary to explain how the system is cleared out at the end of a cycle, since this function is immaterial to an understanding of the present invention and furthermore it has been clearly explained in the above mentioned patents. It will also be understood that the same method of lockout operation is effective for other stations connected to the communication system, no matter how many stations attempt totransmit at the same time, remembering that the station nearest the office having indications ready for transmission will get its lockout relay up and pick up the associated FB relay, for connecting the line to the source of line current at the associated station.

Having thus described one specific embodiment of a centralized traffic controlling system, it is desired to be understood that the particular arrangements illustrated are only typical illustrations of applicants invention and are not intended to illustrate the exact circuit design necessary to carry out the features of the invention, this particular form having been selected to facilitate in the disclosure rather than to limit the number of forms which it may assume.

What I claim is:-

1. In a remote control system; a control oilice and a plurality of field stations connected by a line circuit; a transmitter at each of said stations for transmitting signals over said line circuit when the transmitter is rendered active; a selecting relay at each of-said stations for associating the associated transmitter with the line circuit when the selecting relay is rendered active; a lookout relay at each of said stations having active and inactive positions; means for placing a plurality of said lockout relays in their active positions when the associated stations having signals ready to transmit at the same time; means responsive to the placement of the lookout relay at that one of said stations which is nearest to said ofiice in its active position for energizing said line circuit from the oflice to said nearest station, said last mentioned means including an electric valve allowing the flow of current in a particular direction and to block the flow of current if the polarity of the, source from which it is derived is reversed; means responsive to the energization of said line circuit by the lockout relay at said nearest station for rendering the associated selecting relay active; means jointly controlled by the lookout relay and the selecting relay at said nearest station for rendering the associated transmitter active; and means controlled by the selecting relay at said nearest station for isolating the line extending to a station more remote from said ofiice than said nearest station.

2. In a remote control system, a control oifice and a field station connected by a line circuit including in series therein a rectifier located at said station permitting the flow of current of one polarity and blocking the flow of current of the opposite polarity, a transmitter at said office and a transmitter at said field station for respectively transmitting messages to said station and to said ofiice when rendered active, means in said ofiice for normally applying potential of said opposite polarity to said line circuit, means at said office for at times reversing the polarity of said potential to cause current of said one polarity to flow through said rectifier, means for at times shunting said rectifier to cause the flow of current in said line circuit of opposite'polarity, means responsive to the flow of current in said line circuit of said one polarity for rendering the transmitter at said office active and the transmitter at said station inactive, and means responsive to the flow of current in said line circuit of the opposite polarity for rendering the transmitter at said station active and the transmitter at said ofiice inactive.

3. a remote control system; a control office at' a plurality of field stations connected by a line circuit; a transmitter at said ofiice for transmitting messages over said line circuit to said stations when said transmitter is rendered active; a transmitter at each of said stations for transmitting messages over said line circuit to said oiiice when the transmitter at the associated station is rendered active; a sourcevof direct current :3

connected to'said line circuit in a particular polar direction, an electric valve at the most remote station normally connected in said line circuit in a direction so that current from said source when so connected is prevented from flowing in said line circuit; initiating means for reversing the polar direction of said source of current in said line circuit, whereby current of reverse polarity flows over said line circuit and through said electric valve; means for at times shunting said electric valve to cause flow of current of said particular polarity; means responsive to said current 'of particular polarity to render the transmitter at said office inactive, means responsive to said current of reverse polarity for rendering the transmitters at said stations inactive; and means responsive to said initiating means for rendering the transmitter at said ofiice active.

4. In a remote control system; a control office and a plurality of field stations connected by a line circuit; a transmitter at said office for transmitting messages over said line circuit to said stations when said transmitter is rendered active; a transmitter at each of said stations for transmitting messages over said line circuit to said orfice when thetransmitter at the said station is rendered active; a source of direct current reversibly connected to said line circuit to supply current of normal or reverse polarity, but

normally supplying a potential of normal polarity; an electric valve at the most remote station normally connected in said line circuit in a polar direction so that current from said potential of normal polarity is prevented from flowing; a first initiating means for connecting said source of current to said line circuit in a reverse direction, whereby current of a reverse polarity fiows ply a normal or a reverse polarity, and normally supplying a potential of normal polarity in said 7 line circuit; an electric valve at one of said stations normally connected in said line circuit and so poled that current from the normalconnection of said source of current to said line circuit is prevented from flowing; a first initiating means for reversing the connection of said source of current to said line circuit, whereby reverse current flows over said line circuit and through said. electric valve; means responsive to said reverse current flow for rendering the transmitters atsaid stations inactive; means responsive to said first initiating means for rendering the transmitter at said ofiice active; a second initiating means including an electric valve at each of said stations which operates to connect the electric valve at that station across said line circuit in a poled direction to allow the flow of current of normal polarity over said line circuit but to block the fiow of current of reverse polarity; means responsive to normal current flow over said line circuit for rendering the transmitter at that station active and the transmitter at said ofiice inactive; means momentarily interrupting current flow in said line circuit; and means responsive,

to said current flow interruption for rendering all out one of said station transmitters inactive if and when the second initiating means at more than one station operate simultaneously.

6. In a remote control system, a control oifice and a, plurality of field stations connected by a single line circuit extending from the ofiice to the nearest station and then from station to station to the most remote station, a polar line relay at said office and at each of said stations connected in series in said line circuit, a source of direct current at said ofiice normally applying a potential or" normal polarity to said line circuit, an electric valve at the most remote station included in series in said line circuit in a polar direction to prevent the flow of current of normal polarity in said line circuit, normally inactive transmitting means at said office and including the line relay at the on'ice for transmitting messages over said line circuit to said fieldstations, normally inactive transmitting means at each of said field stations and includingthe line relay at such station for transmitting messages over said line circuit to said ofiice, ofiice initiating means for reversing said source of direct current in said line circuit to cause the fiow of current of reverse polarity over said line circuit and through said electric valve, means responsive to said current of reverse polarity for rendering said transmitting means in said office effective, field station initiating means at each field station for by-passingsaid electric valve to allow the flow of current of normal polarity, means responsive to said current of normal polarity for rendering the associated transmitting means at such field station active, and means for momentarily opening said line circuit after a field station initiating means at a particular field station causes the flow of current in said line circuit and for rendering all other field station initiating means inactive. i. V

7. In a remote control system,'a control office and a plurality of field stations connected by a single line circuit extending from the ofiice to the nearest station'and then from station, to station to the most remote station, a polar line relay at said ofiice and at each of said stations connected in series in said line circuit,,a source of direct current at said ofiice normally applying a potential of normal polarity to said line circuit, an electric valve at the most remote stationincluded in series in said line circuit in a polar direction to prevent the flow of current of normal polarity in said line circuit, normally inactive transmitting means at said ofiice and including the source of direct current and the line relay at the office for transmitting messages over said line circuit to said field stations, normally inactive transmitting means at each of said field stations and, including theline relay and a source of direct current at such station .for transmitting messages over said line circuit to said ofiice, ofiice initiating means for reversing said source of direct current in said line circuit to cause the flow of current of reverse polarity over said line circuit and through said rectifier, means responsive to said current of reverse polarity for rendering said transmitting means in said office eiiective, field station initiating means at each field station for by-passing said electric valve to allow the flow of current of normal polarity, means responsive to said current of normal polarity forrenda ng the associated transmitting means at such field station active, and means for momentarily opening said line circuit after a field station'initiating means at a particular field station causes the fiow of current in said line circuit for first disconnecting said office source from and then connecting the field transmitting means including its source of current to said line circuit.

8. In a remote control system, a control ofiice and a field station connected by a single line circuit extending from the office to said station, a polar line relay at said oifice and at said station connected in series in said line circuit, a source of direct current at said oflicenormally applying a potential of normal polarity to said line circuit, an electric valve at the station included inseries in said line circuit in a polar direction to prevent the flow of current of normal polarity in'said line circuit, normally inactive transmitting means at said office and including the line relay at the omce for transmitting messages over said line cir-' cuit to said field station, normally inactivetrans mitting means at said field station and including the linerelay at such station for transmitting messages over said line circuit to'said office, offic'e initiating means for rendering saidoifice transmitting means active and for reversing said source of direct current in said line circuit to cause the flow of current of reverse polarity over said line circuit and through said electric valve, means responsive to said current of reverse polarity for rendering said transmitting means at said field station inactive, field station initiating means at said field station for by-passing saidielectric valve to allow the flow of current of normal polarity, and means responsive to said current of normal polarity for rendering the associated 

